In a conventional method of synthesizing melody, a melody is first divided into a tone wave and an envelope wave and then the two waves are multiplied together to obtain a synthesized sound wave. This is shown in FIGS. 1.about.3. FIG. 1 shows a conventional tone wave 10 plotted as a voltage vs time curve. An envelope wave shown in FIG. 2 is also plotted as a voltage vs time curve. Multiplying the tone wave 10 in FIG. 1 by the envelope wave 20 in FIG. 2, a composite sound wave 30 is obtained which is shown in FIG. 3. It should be noted that the composite sound wave 30 shown in FIG. 3 is obtained under ideal conditions, i.e. a hypothetical waveform.
In reality, the composite waveform 30 is more likely to have a shape of the directly composite waveform 40 shown in FIG. 4. This departure from the ideal form is mainly caused by the lack of adjustment for the DC voltage offset.
Traditionally, a melody can be synthesized in two ways. The first is a digital synthesizing method in which the digital data of the tone wave and the envelope wave are multiplied together by a multiplier. The product of the multiplication is then sent to a digital/analog converter in order to complete the synthesis of the melody wave. The shortcoming of this method is that it requires the additional component of a multiplier and a higher frequency range for the conversion system.
The second method of synthesizing a melody wave is an analog synthesizing method. However, due to the lack of adjustment for the voltage offset and the lack of adequate positive/negative signal processing of the digital/analog converter, a distorted sound wave is frequently generated. This is shown in FIG. 4. The sound wave 40 shown in FIG. 4 contains a variable DC value, it frequently causes an undesirable "pop" noise during the playback of the melody. In order to eliminate this "pop" noise, at least one coupling capacitor must be used to eliminate the variable DC component. And, a proper bias voltage/current is set to amplify for proper operating. This in turn increases the cost of the circuit.
It is therefore an object of the present invention to provide a digital/analog converter circuit of the analog multiplying type that does not have the shortcomings of the prior art digital/analog converter circuits.
It is another object of the present invention to provide a digital/analog converter circuit of the analog multiplying type that utilizes an analog synthesized circuit.
It is a further object of the present invention to provide a digital/analog converter circuit of the analog multiplying type that utilizes an analog synthesizing method to achieve both cost savings and efficiency improvement.
It is yet another object of the present invention to provide a digital/analog converter circuit of the analog multiplying type utilizing an analog synthesizing method that has the components of a code converter, an analog multiplying current source, and a bridge output circuit.